The present invention relates to capacitor-sensors and, more particularly, to improvements in capacitors formed from patterned electrodes.
The present invention is related to self-adjusting weighing systems for measuring gems and other very light masses and, more particularly, to the system described in my U.S. Pat. No. 4,738,324, entitled "Self-Adjusting Weighing System" issued Apr. 19, 1988 (hereafter '324 patent). This patent is incorporated by reference herein.
There are many ways to measure weight. For top-loading balances, where the mass to be weighed is placed on top of a pan, a common problem is that of off-center pan loading in which the mass to be weighed is not located at the exact center of the weighing pan, but is off to one side. In general, when a load is placed deviatedly from the required point of placement, such as the center of the pan, the bending moment exerted on the pan thereby produces an error in the measured value.
The '324 patent cited above discloses an apparatus for dealing with the problem of off-center pan loading. The apparatus technique is a self-adjusting one in which the effects of off-center loading in the load cell are measured and corrected before the weight reading is outputted. The device disclosed in said patent measures deflection at a plurality of locations on a spring-supported pan, and combines these separate measurements to yield the weight on the pan, corrected for load placement location, i.e., off-center loading. The deflection measurements are accomplished using a plurality of capacitor-sensors formed between two rigid, closely-spaced plates with specific electrode patterns on the plates forming the capacitors. These are not actually three physically separate capacitors, but one device acting like three different capacitors and/or performing as if it were three separate capacitors. The plates are attached to the weight bearing pan and a base. The plates are held apart by three or more springs spaced around the periphery of the plates. The measurements are electronically gathered as measured values for each capacitor.
The capacitor-sensor arrangement used in the '324 patent is illustrated in FIG. 1. The capacitor-sensor has a top plate and a bottom plate. The bottom plate is made of nonconductive material, e.g., ceramic, but has a specific pattern of conductive material silk screened onto its upper surface, consisting of three, separate, symmetrical, roughly one hundred twenty degree, pie-shaped regions. The top plate is also made of nonconductive material. Its undersurface has a solid, three hundred sixty degree conductive region silk screened on. In effect, the two plates form three, pie-shaped capacitor-sensors. The electrical leads for the three pie-shaped regions are brought out between regions #1 and #3 of the lower plate to a point at the circumference of the lower plate. The leaf springs translate the weight of the object being measured into displacement between the plates. The three capacitors formed are employed as sensors in which the weight applied against the sensor affects a change of capacitance which is converted by conventional electronics to a frequency inversely proportional to the capacitance. Thus, electrical outputs are obtainable which are analogs of the weight of the object being measured, the variation in values of the three capacitors reflecting tipping from the position of the object being measured on the pan as well as its weight. In the '324 apparatus perfect centering of the object to be measured on the top pan would mean nominally that the capacitance values for each of the capacitance regions would be the same. However, the slightest deviation from perfect nominal centering would mean that the capacitance values for each would be different. This was the basis for the '324 invention.
To accomplish the requirements of the '324 apparatus, the capacitor-sensors must reflect as directly as possible deviations from perfect centering of the mass to be measured. Because of the lightness of the masses to be measured, even a slight sensitivity of the capacitor-sensors to other factors could distort measurements.
It has been found that the capacitor-sensor configuration of the '324 patent (see FIG. 1) was sensitive to two factors not previously considered. One factor has to do with a contribution of capacitance due to the long lead from the circumference edge contact to the region #2 electrode area between regions #1 and #3. The effective electrode of region #2, therefore, was slightly different in shape and area from electrode regions #1 and #3. This asymmetry has caused problems in linearizing the weight output and correcting for side-to-side errors in the weight output.
The second factor has to do with horizontal motion sensitivity. The bottom plate in the '324 apparatus is fixed. However, the top plate may move laterally to some extent. An example of this would be if the scale is not level and horizontal. During the weighing operation there will be a resultant lateral motion between the upper and bottom electrodes to which sensitivity is not desirable. The result of the lateral movement on capacitance is that it affects the capacitance area, i.e., the resultant area between two electrodes, and changes the capacitance before factors such as distance between electrodes is considered.